Field of Invention
The present invention relates to a flyback power converter circuit. Particularly, it relates to a flyback power converter circuit which includes a secondary side synchronous rectifier switch and is able to reduce the parasitic capacitance coupling effect of the synchronous rectifier switch. The present invention also relates to a secondary side control circuit for use in the flyback power converter circuit.
Description of Related Art
FIG. 1A shows a prior art flyback power converter circuit (flyback power converter circuit 1). A transformer 10 includes a primary side winding W1 which receives an input voltage VIN. A power switch N1 controls the conduction of the primary side winding W1 to generate an output voltage VOUT at the secondary side winding W2. For higher efficiency, a synchronous rectifier (SR) switch N2 is provided at the secondary side of the transformer 10, which is coupled to the secondary side winding W2 to control the conduction time of the secondary side winding W2, such that the secondary side winding W2 is conductive when the primary side winding W1 is not conductive. The secondary side control circuit is located at the secondary side and is coupled to the SR switch N2, for controlling the SR switch N2 according to information of for example but not limited to the output voltage VOUT or the SR switch current.
The prior art circuit in FIG. 1A has a drawback as below. The SR switch N2 has a parasitic capacitance (such as the parasitic capacitance CP between the gate and the drain of the SR switch N2 as shown in the figure). Before the supply voltage VDD of the secondary side control circuit 20 achieves its normal operation level, the secondary side control circuit 20 cannot operate properly, and the SR switch N2 may be erroneously turned on because the control terminal DRV of the SR switch N2 is affected by the coupling effect of the parasitic capacitance CP. The supply voltage VDD is the power supply of the secondary side control circuit 20, which may be obtained for example directly from the output voltage VOUT, or by filtering, dividing or regulating the output voltage VOUT.
Referring to FIG. 1B, before the supply voltage VDD achieves the normal operating threshold VPR, the secondary side control circuit 20 cannot operate properly. At time point Tl, due to the switching of the power switch N1 and the induction between the primary and the secondary side windings W1 and W2, the control terminal DRV of the SR switch N2 is coupled to a high level by the parasitic capacitance CP, causing the power switch N1 and the SR switch N2 to be conductive at the same time, resulting in large current spikes through the power switch N1 and the SR switch N2 to damage these two switches.
FIGS. 2A-2C show several other prior art flyback power converter circuits attempting to solve the problems as addressed. As shown in FIGS. 2A and 2B, the flyback power converter circuits 2A and 2B include respectively a resistor R1 and a capacitor C1 coupled to the control terminal DRV of the SR switch N2. However, the equivalent impedance of R1 or C1 has to be small to an extent that the coupling effect can be reduced effectively, but the small impedance may cause extra power consumption, slow transient, or efficiency loss. The flyback power converter circuit 2C shown in FIG. 2C includes an SR switch N3 whose threshold voltage is higher (the threshold voltage of N3>the threshold voltage of N1, i.e. Vth (N3)>Vth(N1)) to avoid the aforementioned problems. However, this prior art has a higher manufacturing cost and lower efficiency.
Compared to the prior art circuits in FIGS. 1A and 2A-2C, the present invention is advantageous in that the aforementioned parasitic coupling effect can be reduced without increasing cost or efficiency loss.